G Model ARTICLE IN PRESS to achieve the desired temperature of the test specimen. The deter- lined power settings were then used in actual tests. Fracture surfaces of failed specimens were examined us optical microscope( Zeiss Discovery V12) Density and porosity of the as-processed composite as well as of the specimens subjected to prior thermo-mechanical loading were measured using a Micro- metrics Auto Pore Ill Mercury Porosimeter. The porosimeter was calibrated using a standard blank in accordance with the proce- dures in ASTM Standard D 4284-03 and ASTM Standard D 4404-84 0.0025MPa/s ( Reapproved 2004). Both low and high pressure runs were carried out. The average volume of material analyzed using porosimetry was 0.5 cm. In the case of the specimens with prior therme mechanical loading history, the measurements were performed on the material cut from the gage section of the test specimen. Prior to measurements, the samples were heat treated at 100 C for 20 min STRAIN(%) in order to remove any excess moisture that may have accumulated in the material. The pore volume and the pore volume distribution Fig. 2. Tensile stress-strain curves for N720/A ceramic vere determined using procedures in the aforementioned ASTM 1200 C with the stress rates of 25 and 0.0025 MPa/ s. The it on stress-strain behavior Standards 3. Test procedures from 0. 1 to 100 MPa/s. A linear stress-strain behavior was produced only at the fastest loading rate of 1000 MPa/s. These results indicate all mechanical tests were performed at 1200C in laboratory that at stress rates <1000 MPa/s the tensile behavior and tensile air. Each test specimen was heated to 1200 C in 25 min, and held strength ov The nonlinear stress-strain behavior of the N720/A pecimens of 152-mm total length with a 10-mm-wide gage sectio composite produced at 25 MPa/s in the present study likewise sug- were used in all tests. Monotonic tension tests were performed in gests that creep deformation and damage mechanisms affect the load control with the constant rates of 0.0025 and 25 MPa/s Creep- monotonic tensile behavior of this CMC at 1200oC. rupture tests were conducted in load control in accordance with As seen in Fig. 2, the tensile stress-strain behavior produced ASTM Standard C 1337. Specimens were loaded to the creep stress at the stress rate of 0.0025 MPa/s is fundamentally different from 25 MPa s. The stress-strain curves level at the rate of 25 MPa)s In each test, stress-strain data were obtained at 0.0025 MPa/s are markedly nonlinear.Furthermore,a creep period. Thus both total strain and creep strain could be calcu- non-monotonic change in strain is observed in all 0.0025 MPa/s tests As the stress increases the strain first decreases and then lated and examined. Creep run-out was defined as 100 h at a given begins to increase. The corresponding rate of change of strain is creep stress, which is consistent with the service life expected n first negative and then becomes positive. Such strain rate reversal etained strength and modulus, specimens that achieved run-out occurs as the stress reaches 20-25 MPa. as the stress continues to were subjected to tensile tests to failure at 25 MPa/s at 1200 C In rease, appreciable inelastic strains develop The failure strains some ca anging from 0.73 to 1.06%, are two to three times those obtained at ne specimen was tested per test condition. The authors 25 MPa/s. Conversely, the average UTS of 154 MPa is considerably recognize that this is a limited set of data. However, extreme care lower than the strength values obtained at 25 MPa/ s. was taken in generating the data. Selective duplicate tests have demonstrated the data to bevery repeatable. This exploratory effort at 25 and 0. 0025 MPa/s are shown in Fig 3(a)and(b),respectively serves to identify the behavioral trends and to determine whethe a more rigorous investigation should be undertaken. The fracture planes of both specimens are not well defined. The 0 fiber tows break over a wide range of axial locations, in general spanning the entire width of the specimen. The fibers in the 0 tows 4. Results and discussion in each cloth layer exhibit random failure producing brushy fracture surfaces. Note that the specimen tested at 0.0025 MPa/s has a con- 4.1. Monotonic tension-effect of loading rate siderably longer damage zone (8 mm) than the en tested at 25 MPa/s(5 mm), which accounts for larger strain accumulated The effect of loading rate on tensile stress-strain behavior of at the slower stress rate. the n720/A ceramic composite at 1200Cis typified in Fig. 2. At the The strong dependence of tensile strength on loading rate exhib- stress rate of 25 MPa/s, the average ultimate tensile strength was ited by this composite at 1200 C is similar to that observed at 181 MPa, the average elastic modulus, 70 GPa, and the average fail- elevated temperatures for advanced ceramics 38, 39, 49-51 as well ure strain,0.36%. These results are consistent with the data reported as for the Nextel M720 fibers [52, 47, 48. For composites with a glass earlier [41, 46]. The tensile stress-strain curves obtained at 25 MPa s or glass-ceramic matrix, the degradation of strength with decreas- appear to be linear to failure. Yet, upon closer examination it is seen ing loading rate is a consequence of slow crack growth process due at the stress-strain behavior becomes nonlinear as the stress to stress corrosion 36. However, in the case of oxide ceramics creep reaches 70 MPa, although the nonlinearity is not strongly pro- deformation can have a significant influence on the monotonic ten- nounced. Because of the inherent nature of an exceptionally weak sile behavior. Wilson et al. [52] suggested that the deformation porous matrix, much of the stress-strain behavior of the composite mechanism operat N720 fibers at elevated temperatures was is controlled by the fibers. Material exhibits typical fiber-dominated diffusion creep controlled by interface reaction. As the diffusion composite behavior. Milzet al [47 and Goering and Schneider 48 creep is a time-dependent process, the loading rate dependence studied mechanical behavior of Nextel M720 fibers at temperatures of the N720/A tensile behavior and tensile strength at 1200.C is in the 900-1200 Crange In tensile tests conducted at 1200 C non- readily explained. At slower loading rates, more time is allowed linear stress-strain behavior was observed at stress rates ranging for diffusion creep to develop, leading to extension of existing crit Please cite this article in press as: M.B. Ruggles-Wrenn, et al., Mater. Sci. Eng. A(2008). doi: 10. 1016/j. msea. 2008.03.006Please cite this article in press as: M.B. Ruggles-Wrenn, et al., Mater. Sci. Eng. A (2008), doi:10.1016/j.msea.2008.03.006 ARTICLE IN PRESS G Model MSA-24026; No. of Pages 7 M.B. Ruggles-Wrenn et al. / Materials Science and Engineering A xxx (2008) xxx–xxx 3 to achieve the desired temperature of the test specimen. The deter￾mined power settings were then used in actual tests. Fracture surfaces of failed specimens were examined using an optical microscope (Zeiss Discovery V12). Density and porosity of the as-processed composite as well as of the specimens subjected to prior thermo-mechanical loading were measured using a Micro￾metrics Auto Pore III Mercury Porosimeter. The porosimeter was calibrated using a standard blank in accordance with the proce￾dures in ASTM Standard D 4284-03 and ASTM Standard D 4404-84 (Reapproved 2004). Both low and high pressure runs were carried out. The average volume of material analyzed using porosimetry was 0.5 cm3. In the case of the specimens with prior thermo￾mechanical loading history, the measurements were performed on the material cut from the gage section of the test specimen. Prior to measurements, the samples were heat treated at 100 ◦C for 20 min in order to remove any excess moisture that may have accumulated in the material. The pore volume and the pore volume distribution were determined using procedures in the aforementioned ASTM Standards. 3. Test procedures All mechanical tests were performed at 1200 ◦C in laboratory air. Each test specimen was heated to 1200 ◦C in 25 min, and held at 1200 ◦C for additional 15 min prior to testing. Dog bone shaped specimens of 152-mm total length with a 10-mm-wide gage section were used in all tests. Monotonic tension tests were performed in load control with the constant rates of 0.0025 and 25 MPa/s. Creep￾rupture tests were conducted in load control in accordance with ASTM Standard C 1337. Specimens were loaded to the creep stress level at the rate of 25 MPa/s. In each test, stress–strain data were recorded during the loading to the creep stress level and the actual creep period. Thus both total strain and creep strain could be calcu￾lated and examined. Creep run-out was defined as 100 h at a given creep stress, which is consistent with the service life expected in aerospace applications at that temperature [45]. To determine the retained strength and modulus, specimens that achieved run-out were subjected to tensile tests to failure at 25 MPa/s at 1200 ◦C. In some cases one specimen was tested per test condition. The authors recognize that this is a limited set of data. However, extreme care was taken in generating the data. Selective duplicate tests have demonstrated the data to be very repeatable. This exploratory effort serves to identify the behavioral trends and to determine whether a more rigorous investigation should be undertaken. 4. Results and discussion 4.1. Monotonic tension—effect of loading rate The effect of loading rate on tensile stress–strain behavior of the N720/A ceramic composite at 1200 ◦C is typified in Fig. 2. At the stress rate of 25 MPa/s, the average ultimate tensile strength was 181 MPa, the average elastic modulus, 70 GPa, and the average fail￾ure strain, 0.36%. These results are consistent with the data reported earlier [41,46]. The tensile stress–strain curves obtained at 25 MPa/s appear to be linear to failure. Yet, upon closer examination it is seen that the stress–strain behavior becomes nonlinear as the stress reaches ∼70 MPa, although the nonlinearity is not strongly pro￾nounced. Because of the inherent nature of an exceptionally weak porous matrix, much of the stress–strain behavior of the composite is controlled by the fibers.Material exhibits typical fiber-dominated composite behavior. Milz et al. [47] and Goering and Schneider [48] studied mechanical behavior of NextelTM720 fibers at temperatures in the 900–1200 ◦C range. In tensile tests conducted at 1200 ◦C non￾linear stress–strain behavior was observed at stress rates ranging Fig. 2. Tensile stress–strain curves for N720/A ceramic composite obtained at 1200 ◦C with the stress rates of 25 and 0.0025 MPa/s. The influence of loading rate on stress–strain behavior is evident. from 0.1 to 100 MPa/s. A linear stress–strain behavior was produced only at the fastest loading rate of 1000 MPa/s. These results indicate that at stress rates <1000 MPa/s the tensile behavior and tensile strength of the N720 fibers are strongly influenced by creep damage mechanisms. The nonlinear stress–strain behavior of the N720/A composite produced at 25 MPa/s in the present study likewise sug￾gests that creep deformation and damage mechanisms affect the monotonic tensile behavior of this CMC at 1200 ◦C. As seen in Fig. 2, the tensile stress–strain behavior produced at the stress rate of 0.0025 MPa/s is fundamentally different from the nearly linear response at 25 MPa/s. The stress–strain curves obtained at 0.0025 MPa/s are markedly nonlinear. Furthermore, a non-monotonic change in strain is observed in all 0.0025 MPa/s tests. As the stress increases, the strain first decreases and then begins to increase. The corresponding rate of change of strain is first negative and then becomes positive. Such strain rate reversal occurs as the stress reaches 20–25 MPa. As the stress continues to increase, appreciable inelastic strains develop. The failure strains, ranging from 0.73 to 1.06%, are two to three times those obtained at 25 MPa/s. Conversely, the average UTS of 154 MPa is considerably lower than the strength values obtained at 25 MPa/s. Optical micrographs of fracture surfaces obtained in tensile tests at 25 and 0.0025 MPa/s are shown in Fig. 3(a) and (b), respectively. The fracture planes of both specimens are not well defined. The 0◦ fiber tows break over a wide range of axial locations, in general spanning the entire width of the specimen. The fibers in the 0◦ tows in each cloth layer exhibit random failure producing brushy fracture surfaces. Note that the specimen tested at 0.0025 MPa/s has a con￾siderably longer damage zone (∼8 mm) than the specimen tested at 25 MPa/s (∼5 mm), which accounts for larger strain accumulated at the slower stress rate. The strong dependence of tensile strength on loading rate exhib￾ited by this composite at 1200 ◦C is similar to that observed at elevated temperatures for advanced ceramics [38,39,49–51] as well as for the NextelTM720 fibers [52,47,48]. For composites with a glass or glass-ceramic matrix, the degradation of strength with decreas￾ing loading rate is a consequence of slow crack growth process due to stress corrosion[36]. However, in the case of oxide ceramics creep deformation can have a significant influence on the monotonic ten￾sile behavior. Wilson et al. [52] suggested that the deformation mechanism operating in N720 fibers at elevated temperatures was diffusion creep controlled by interface reaction. As the diffusion creep is a time-dependent process, the loading rate dependence of the N720/A tensile behavior and tensile strength at 1200 ◦C is readily explained. At slower loading rates, more time is allowed for diffusion creep to develop, leading to extension of existing crit-